Variable valve activation mechanisms for internal combustion engines are well known. It is known to lower the lift, or even to provide no lift at all, of one or more valves of an internal combustion engine, during periods of light engine load. Such valve deactivation or valve lift switching can substantially improve fuel efficiency.
A rocker arm acts between a rotating eccentric camshaft lobe and a pivot point on the internal combustion engine, such as a hydraulic lash adjuster, to open and close an engine valve. Switchable rocker arms may be a “deactivation” type or a “two-step” type. The term switchable deactivation rocker arm, as used herein, means the switchable rocker arm is capable of switching from a valve lift mode to a no lift mode. The term switchable two-step rocker arm, as used herein, means the switchable rocker arm is capable of switching from a first valve lift mode to a second and lesser valve lift mode, that is greater than no lift. It should be noted that the second valve lift mode may provide one or both of decreased lift magnitude and decreased lift duration of the engine valve compared to the first valve lift mode. When the term “switchable rocker arm” is used herein, by itself, it includes both types.
A typical switchable rocker arm includes an outer arm and an inner arm where the inner arm includes an inner arm follower which follows a first profile of a camshaft of the internal combustion engine and where the outer arm includes a pair of outer arm followers which follow respective second and third profiles of the camshaft. The follower of the inner arm and the followers of the outer arm may be either sliding surfaces or rollers and combinations thereof. The inner arm is movably connected to the outer arm and can be switched from a coupled state wherein the inner arm is immobilized relative to the outer arm, to a decoupled state wherein the inner arm can move relative to the outer arm. Typically, the outer arm of the switchable rocker arm is pivotally supported at a first end by the hydraulic lash adjuster which fits into a socket of the outer arm. A second end of the outer arm operates against an associated engine valve for opening and closing the valve by the rotation of an associated eccentric cam lobe acting on the follower of the inner arm. The inner arm is connected to the outer arm for pivotal movement about the outer arm's second end with the follower of the inner arm disposed between the first and second ends of the outer arm. Switching between the coupled state and the decoupled state is accomplished through a lock pin which is slidingly positioned in a lock pin bore of the outer arm. One end of the lock pin is moved into and out of engagement with the inner arm. Consequently, when the lock pin is engaged with the inner arm, the coupled state is achieved. Conversely, when the lock pin is not engaged with the inner arm, the decoupled state is achieved. As shown in U.S. Pat. No. 7,305,951 to Fernandez et al., the disclosure of which is hereby incorporated by reference in its entirety, the other end of the lock pin acts as a piston upon which pressurized oil is applied and vented to affect the position of the lock pin. Also as shown by Fernandez et al., oil is supplied to the lock pin via an oil supply bore which originates in the socket and breaks into the lock pin bore.
Variations in manufacturing of the various components of the switchable rocker arm lead to varying magnitudes of lash between the lock pin and the inner arm where the lash is the distance between the lock pin and the surface of the inner arm which engages the lock pin when the inner follower is engaged with the base circle of the camshaft. Negative lash, i.e. interference, prevents the lock pin from moving from the decoupled state to the coupled state while excessive lash affects the valve lift when the lock pin is in the coupled state. In order to provide a desired magnitude of lash, it is known to use a manufacturing process which provides a plurality of inner followers, typically in the form of a roller, of various known sizes, where this process is typically called zoning. During manufacturing, the lash is observed. If the lash falls outside of the desired tolerance range, a different inner follower is selected from the plurality of different sizes and the original inner follower is replaced in order to bring the lash into the desired tolerance range. Alternatively, it is also known to use this same process, except with the outer followers, to bring the lash into the desired tolerance range. While this process may be effective, it adds complexity and time to the manufacturing process and also increases costs since the inner followers must be zoned.
What is needed is a rocker arm which minimizes or eliminates one or more of the shortcomings as set forth above.